When the recent monkey pox (mpox) outbreak first began in the United States in May 2022, public health officials suggested that childhood smallpox vaccination might confer some cross-protection.

It remained unclear, however, if that was adequate. “Although persons aged >50 years likely received a childhood smallpox vaccination, they might have more comorbidities and a higher risk for severe mpox compared with those aged ≤50 years. Information about waning cross-protective immunity and how this might affect risk for severe mpox is limited,” according to a recent article in the Morbidity & Mortality Weekly Report.

The researchers from the CDC analyzed mpox cases among 29,984 adults, finding that those aged 50 years and older had a higher prevalence of immunocompromising conditions and HIV but a lower prevalence of symptoms than those aged younger than 50 years.

“Among 1,020 adults aged >50 years with vaccination data, prevalences of pruritus, constitutional symptoms, and hospitalization were lower among those who received Jynneos vaccine than among those who had not,” the authors point out.

The authors advised that “all adults at risk for mpox should receive Jynneos vaccine, irrespective of childhood smallpox vaccination status.”

From May 2022 to May 2023, approximately 30,000 mpox cases were reported in the U.S., predominantly among young adult men. In fact, slightly less than 10% were aged 50 years or older, and most of those were cisgender men.

The report noted that adults aged older than 50 years were likely to have received a childhood smallpox vaccination, which was given routinely to children in the U.S. prior to being discontinued in 1971. It added that the childhood smallpox vaccination has been shown to provide partial protection against mpox—both in preventing monkeypox virus infection and severe mpox—in previous studies.

The study found, however, that immunocompromising conditions were more prevalent overall among adults aged older than 50 years (15.0%) than among those aged 18 to 50 years (11.1%), as was receipt of Jynneos vaccine (27.6% and 21.3%, respectively). “Examination of symptoms and outcomes among adults with mpox who had not received Jynneos vaccine found lower prevalences of gastrointestinal (rectal or abdominal) and constitutional symptoms among those aged >50 years (37.3% and 85.0%, respectively) than among those aged 18–50 years (50.3% and 91.2%, respectively); the prevalences of hospitalization and death were comparable between the two age groups.”

The researchers concluded that mpox might affect multiple organ systems so patients with comorbidities, including HIV and immunocompromising conditions, which are more prevalent among older adults, might be at increased risk for more severe mpox disease. They advised that most deaths and hospitalizations among adults with mpox in the U.S. during the recent outbreak occurred in persons with immunocompromising conditions.

“Although childhood smallpox vaccination might confer some protection against monkeypox virus infection and might mitigate mpox disease severity, it is likely that receiving the currently recommended Jynneos vaccine provided additional protection,” the authors emphasized, adding, “This finding underscores the importance that persons at risk for mpox, particularly adults aged >50 years, receive currently recommended Jynneos,” pointing out that the immunity might have waned.

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